This invention relates to machine vision and, more particularly, to systems for illuminating an object.
Machine vision typically involves illuminating an object, e.g., a semiconductor chip, and capturing an image, by video camera, using the light reflected off the object for computer analysis. Among the prior art illumination techniques are those of "front" lighting and "back" lighting. According to the former, the object is illuminated from the front, e.g., by a small spotlight or flood, and the image is captured for analysis. According to the latter, the object is illuminated from behind and its silhouette is captured by the camera.
A problem with basic implementations of front lighting techniques is that they fail to adequately illuminate some features of the object and, consequently, to enable those features to be captured by the camera. In a front lighting system, some features (or defects) may be positioned or angled in such a way that light reflected off them does not reach the camera. Thus, for example, light reflected off a beveled specular edge may "miss" the camera lens, effectively hiding that feature and preventing its further analysis. Of course, since a back-lighting system illuminates from the rear, features on the face of the object will not be captured.
One prior art front lighting technique, employed in the photographic arts, uses a variant of a continuous hemispherical illuminator to illuminate an object. Here, the object is surrounded by a tent of cloth illuminated uniformly from the outside. A camera lens is placed in a small hole in the cloth to permit the object image to be captured on film.
A problem with this type of frontal illumination is that it tends to produce a dark spot on the object at a position opposite the hole. To minimize that spot, the hole must be small with respect to the distance to the object, requiring a large tent and long focal length lens.
An object of this invention to simulate such tent illumination in a very small space and with lenses that are large with respect to the distance to the object.
Other general and more specific objects of this invention will in part be obvious and evident from the drawings and description which follows.